1. Field of the Invention
The field of art to which this invention relates is display devices. It is more particularly directed to color sequential liquid crystal displays.
2. Description of the Related Art
Color projection displays based on liquid crystal technology require a scheme to control the three primary colors necessary to produce an image. One implementation calls for the use of three liquid crystal spatial light modulators (SLMs), one for each color. However, the optics required for splitting and recombining the three colors leads to a complicated and costly system that is difficult to fit in a compact display.
The number of SLMs, or light valves, can be reduced if color sequencing is used. In the color sequential mode of operation, the red, green and blue components are imaged sequentially in time at three times the typical rate (e.g., 3.times.60 Hz=180 Hz).
As long as the three color images are presented at a fast enough rate to the viewer, the time-averaged image of the three individual colors appears as the composite full-color image.
A simplified, typical configuration for a single light valve display is shown in FIGS. 1A and 1B. It consists of a white light lamp source 18 combined with a color filter wheel 16 which sequentially alternates the red, green and blue primary colors 16a, 16b, 16c respectively. In the reflective SLM mode illustrated in FIG. 1A, a polarizing beamsplitter cube 20 directs one polarization of each color component onto the SLM 12. The SLM 12 will rotate the polarization of the selected pixels to create that color component of the image. The image formed by the modulated polarization will be reflected by the SLM 12 and will pass through the beamsplitter cube 20. A lens 22 is then used to magnify this image and project it onto a screen 24. The process is then repeated for the other two color components.
For the transmission SLM mode illustrated in FIG. 1B, each color component is directed onto a transmission liquid crystal panel 14 which is placed between two polarizing films 26, 28 that are oriented 90.degree. to one another. Light will be transmitted through the cross polarizers 26, 28 only for pixels that have modulated the incoming polarization by the transmissive liquid crystal panel 14. The transmitted modulated light will then be imaged by a projection lens 22 onto the screen 24.
In order to maintain high optical quality in the image, a sophisticated lens 22 must be manufactured which works well for all three colors. In particular, the magnification of the lens 22 must be identical for the three colors to ensure pixel overlay over the entire image. A totally achromatic projection lens 22 is difficult to design and expensive to fabricate.
The polychromatic nature of the light source presents an additional problem in the operation of the liquid crystal cell itself. Typically, the type of liquid crystal material and the thickness of the liquid crystal layer in the cell can be optimized to produce a 90.degree. polarization change for only one color. For the other two colors, the polarization rotation will not be complete. Incomplete polarization rotation will result in lower throughput, chromatic gray scale variations and loss of contrast.